This application claims priority to an application entitled xe2x80x9cLiquid Electrophotographic Printing Apparatusxe2x80x9d filed in the Republic of Korea on May 15, 2000, and assigned Application No. 00-25768, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid electrophotographic printing apparatus, and more particularly, to a liquid electrophotographic printing apparatus having an improved structure so that differences in peeling forces of a photosensitive belt, a transfer roller, and a fuser roller which occur during image transfer, can be controlled to be within a predetermined range.
2. Description of the Related Art
In general, in a liquid electrophotographic printing apparatus, a laser beam scans a photosensitive medium to form an electrostatic latent image on the photosensitive medium, a liquid developer having a predetermined color is applied to the electrostatic latent image area of the photosensitive medium to develop the electrostatic latent image, and then the developed image is printed on a sheet of paper when the image is transferred to the paper via a transfer unit.
Referring to FIG. 1, in a conventional liquid electrophotographic printing apparatus, electrostatic latent images formed on a photosensitive belt 10 by laser scanning units are developed at developing units 20, sequentially and respectively, and then carrier included in the developed image is dried by a drying unit 30, and the image is transferred to a paper sheet (P) via a transfer unit 40.
The transfer unit 40 comprises a transfer roller 41 which is installed to face a transfer backup roller 12 and to which the image (I) developed on the photosensitive belt 10 is transferred, and a fuser roller 43 disposed to face the transfer roller 41. Here, the image transfer from the photosensitive belt 10 to the transfer roller 41 is referred to as transfer T1, and the image transfer from the transfer roller 41 to the paper sheet (P) is referred to as transfer T2.
In the liquid electrophotographic printing apparatus configured as described above, whether or not the developed image is transferred from one device to another is determined by differences in surface energies of the photosensitive belt 10, the transfer roller 41, the paper sheet (P), and the fuser roller 43. The surface energy varies with not only materials of components but also heat and pressure developed during the transfer operation. That is, since the toner constituting an image is transferred from a component having a smaller surface energy to another having a larger surface energy, the materials of individual components and pressing forces between the components are decided by considering those factors. Here, the surface energy acts as a factor deciding the surface adhering force of the toner particle, and the surface energies of the components are relatively decided by measuring the peeling forces (g/inch) of the respective components which can be measured. Peeling force is the force required to peel off a measuring tape attached to a component such as the photosensitive belt 10, the transfer roller 41, the fuser roller 43, and the like, and is a relative value which depends on the type of adhesive tape for measuring a peeling force, the pressing force applied when the adhesive tape is attached to an objective component, the speed of a peeling force measuring apparatus, the ambient temperature of the measuring apparatus, and the like.
Since the peeling force measuring apparatus is described in U.S. application Ser No. 09/666,805 entitled xe2x80x9cElectrophotographic Printing Apparatus and Image Transferring Method for an Electrophotographic Printing Apparatusxe2x80x9d, filed by the applicant of this application, a detailed description thereof will be omitted. The contents of U.S. application Ser. No. 09/666,805 are hereby incorporated by reference.
When the peeling forces required at the photosensitive belt 10, the transfer roller 41, and the fixing roller 43 were measured with the same conditions by the above measuring method according to the number of printed paper sheets, it was found that the peeling force increased as the number of printed sheets of paper increased in a general liquid electrophotographic printing apparatus, as shown by the graph in FIG. 2.
Here, the peeling force required at the sheet of paper varies depending on the manufacturer and the use of the printed sheet of paper, and has a constant value in the range of about 600 to 800 g/inch.
Referring to FIGS. 1 and 2, in the transfer T1, since the difference in the peeling forces of the photosensitive belt 10 and the transfer roller 41 is not large at an initial stage (area S1 of FIG. 2), the toner image (I) developed on the photosensitive belt 10 is not completely transferred to the transfer roller 41. The peeling force of the photosensitive belt 10 is increased by the toner of the remaining toner image on the photosensitive belt 10. Accordingly, the efficiency of image transfer from the photosensitive belt 10 to the transfer roller decreases.
Also, in the transfer T2, as the difference in the peeling forces of the transfer roller 41 and the sheet of paper (P) decreases gradually in area S2 as the number of printed sheets increases, the toner image is not completely transferred from the transfer roller 41 to the sheet of paper (P), and remains on the transfer roller 41. Accordingly, as the toner continues to accumulate on the transfer roller 41, the peeling force of the transfer roller 41 increases. In addition, since the fuser roller 43 continues to have a peeling force smaller than that of the transfer roller 41, the toner accumulated on the transfer roller 41 cannot easily move to the fuser roller 43, and the toner continues to accumulate on the transfer roller 41 resulting in poor transfer.
In addition, the peeling force of the transfer roller 41 increases beyond 500 g/inch when the number of printed sheets is greater than 2000 in area S3, and therefore the difference in the peeling forces of the sheet of paper (P) and the transfer roller 41 begins to decrease. In addition, since the peeling force of the employed toner itself is about 600 g/inch, there is a problem in that the sheet of paper (P) wraps around the transfer roller 41, and cannot be discharged to the outside to result in a paper jam in the printer.
To solve the above problems, it is an objective of the present invention to provide an electrophotographic printing apparatus adapted to set up the relationships of the peeling forces required at a photosensitive belt, a transfer roller, and a fuser roller, and to adjust the peeling force of the transfer roller to decrease the possibility of a bad image transfer.
Accordingly, to achieve the above objective, there is provided a liquid electrophotographic printing apparatus including: a photosensitive belt circulating around a predetermined track; a transfer roller to which an image developed by developing units on the photosensitive belt is transferred while the transfer roller contacts and is rotated by the photosensitive belt; a fuser roller providing a pressing force for the transfer roller so that the toner image transferred to the transfer roller can be transferred to a sheet of paper passing through between the transfer roller and the fuser roller; a fuser roller cleaning device contacting the fuser roller continuously or intermittently and maintaining the peeling force of the fuser roller so that the value of the peeling force of the fuser roller can be controlled to within a predetermined range; and a peeling force adjusting device for maintaining the peeling force of the transfer roller so that the value of the peeling force of the transfer roller can be controlled to within a predetermined range, wherein the surface energies SET, SEP, and SEF of the transfer roller, the paper sheet, and the fuser roller, respectively, satisfy the following inequality
SET less than SEP less than SEF. 